1. Field of the Invention
The present invention relates to a fluid control device such as a flow rate control valve for a mass flow controller, a control valve for various kinds of fluid and a micro-flow rate vacuum valve.
2. Description of the Prior Art
A valve body of a valve of this type has been driven manually, electro-magnetically, pneumatically or by means of a motor. However, since a driving portion is larger than a valve body in construction, it has been very difficult to miniaturize a valve as a whole. Also there has been a defect in that movable portions have a shorter useful life-time.
In order to solve this problem, a "thermally driven valve" has been proposed (for example, Japanese Patent Publication No. 59-28794). Referring to FIG. 11 showing one construction example of such a thermally driven valve, 250 designates a base body on which a cylindrical member 251 stands. 252 designates a valve seat provided between a fluid inlet port 253 and a fluid outlet port 254 formed in the base body 250, with 255 and 256 each designating a fluid passage. 257 designates a guide cylinder provided in the cylindrical member 251, said guide cylinder 257 being provided with a thermally expandable body 259 having a heater 258 therewithin. Said thermally expandable body 259 is provided with a valve body 260 for controlling the opening of a valve mouth 252a of the valve seat 252 at the lower end thereof. On heating the thermally expandable body 259 by supplying electrical power to the heater 258, the thermally expandable body 259 is expanded and the opening of the valve mouth 252a is controlled by axial movement of the valve body 260.
However, with a thermally driven valve constructed in the above described manner, since the thermally expandable body 259 is heated by the heater 258 in order to be expanded, disadvantages have occurred in that it takes a long period of time for the thermally expandable body 259 to be expanded to the appointed degree and thus a fast response can not be achieved. Also, the expansion degree of the thermally expandable body 259 is dependent upon an ambient temperature and in order to improve heat conductance, it is impossible to provide too large a diameter to the thermally expandable body 259 and thus it is very flexible. As a result the control accuracy of the opening of the valve mouth 252a is not high, and since the pressure exerted by the valve body 260 due to the expansion of the thermally expandable body 259 is not very large, the valve is unsuitable for applications requiring fluid control of a large flow rate.